The present invention relates to a process for producing a catalyst using a compound recovered from a used catalyst, as a material for elements constituting the catalyst to be produced.
A catalyst containing at least molybdenum, an A element (at least one element selected from the group consisting of phosphorus and arsenic) and an X element (at least one element selected from the group consisting of potassium, rubidium and cesium) can be used for production of methacrylic acid by gas phase catalytic oxidation of methacrolein, production of methacrylic acid by oxidative dehydration of isobutyric acid, and other purpose.
In JP-A-53-113790 and JP-A-63-130144 are described processes for producing a catalyst using, as a material, a compound obtained by treating, with ammonia solution or the like, a used catalyst having a composition such as mentioned above, which used catalyst has been used for production of methacrylic acid by gas phase catalytic oxidation of methacrolein.
However, the catalysts produced by the processes described in JP-A-53-113790 and JP-A-63-130144 are different in structure from virgin catalysts produced by an ordinary process and therefore have shown low catalytic performances in some cases.
Hence, in order to effectively utilize a used catalyst containing at least molybdenum, an A element (at least one element selected from the group consisting of phosphorus and arsenic) and an X element (at least one element selected from the group consisting of potassium, rubidium and cesium), the present invention aims at providing a process for producing a catalyst using, as a material, a compound containing at least molybdenum and said A element, which has been recovered from a used catalyst having a composition such as mentioned above.
The gist of the present invention lies in a process for producing a catalyst, which comprises dispersing, in water, a used catalyst containing at least molybdenum, an A element (at least one element selected from the group consisting of phosphorus and arsenic) and an X element (at least one element selected from the group consisting of potassium, rubidium and cesium), adding thereto an alkali metal compound and/or ammonia solution, then adjusting the resulting mixture to pH 6.5 or less to generate a precipitate containing at least said molybdenum and said A element, and using the precipitate as a material for catalyst-constituting elements.
According to the present invention, it is possible to produce a catalyst using, as a material, a compound which has been recovered from a used catalyst containing at least molybdenum, an A element and an X element and which contains at least molybdenum and the A element; thereby, the used catalyst can be utilized effectively.
The present invention is particularly useful in producing a catalyst of a formula (1) (shown later) for production of methacrylic acid by gas phase catalytic oxidation of methacrolein, using a material recovered from a used catalyst which was, before the use, a catalyst of the formula (1) for production of methacrylic acid by gas phase catalytic oxidation of methacrolein.
According to the present invention, in recovering the compound containing at least molybdenum and the A element, molybdenum and the A element can be recovered at high ratios; therefore, the used catalyst can be utilized effectively.
In the present invention, the used catalyst containing at least molybdenum, an A element and an X element includes catalysts which have been used in, for example, production of methacrylic acid by gas phase catalytic oxidation of methacrolein or production of methacrylic acid by oxidative dehydrogenation of isobutyric acid. A catalyst for use in production of methacrylic acid preferably has a composition represented by the following formula (1):
AaMobVcCudDeXfYgZhOixe2x80x83xe2x80x83(1)
(wherein Mo, V, Cu and O are molybdenum, vanadium, copper and oxygen, respectively; A is at least one element selected from the group consisting of phosphorus and arsenic; D is at least one element selected from the group consisting of antimony, bismuth, germanium, zirconium, tellurium, silver, selenium, silicon, tungsten and boron; X is at least one element selected from the group consisting of potassium, rubidium and cesium; Y is at least one element selected from the group consisting of iron, zinc, chromium, magnesium, tantalum, manganese, cobalt, barium, gallium, cerium and lanthanum; Z is sodium and/or thallium; a, b, c, d, e, f, g, h and i are each the atomic ratio of each element; when b is 12, a=0.5 to 3, c=0.01 to 3, d=0 to 2, e=0 to 3, f=0.01 to 3, g=0 to 3, h=0 to 3, and i is the atomic ratio of oxygen necessary for satisfying the valency of each component other than oxygen).
The used catalyst containing at least molybdenum, an A element and an X element is first dispersed in water. Thereto is added an alkali metal compound and/or ammonia solution. The amount of the alkali metal compound and/or ammonia solution added may be such that the molybdenum, A element and X element are dissolved; however, the amount is preferably such that the resulting mixture has a pH of 8 or more, more preferably such that the mixture has a pH of 8.5 to 12. The alkali metal compound usable has no particular restriction as to the kind; however, there can be mentioned, for example, sodium hydroxide, potassium hydroxide, cesium hydroxide and sodium hydrogencarbonate, with sodium hydroxide being preferred particularly.
Next, to the mixture after addition of the alkali metal compound and/or ammonia solution is added an acid for pH adjustment to 6.5 or less. It is preferable that, prior to the pH adjustment, the insolubles in the mixture are removed by filtration or the like. As the acid added for pH adjustment, there can be mentioned, for example, hydrochloric acid, nitric acid and sulfuric acid, with hydrochloric acid and nitric acid being preferred particularly.
The material after pH adjustment is preferably kept for a given time for formation of a precipitate. The time of keeping is preferably about 0.5 to 24 hours, and the liquid temperature is preferably about normal temperature to 90xc2x0 C. During the keeping, the material may be allowed to stand but preferably is stirred.
The precipitate formed by pH adjustment, i.e. the compound containing at least molybdenum and an A element is presumed, from the compositional analysis and X-ray diffractometry, to contain, as the main component, a Dawson type heteropolyacid salt having a central element (e.g. phosphorus) and molybdenum ratio of 2:18, or a mixture of a Keggin type heteropolyacid salt having a central element (e.g. phosphorus) and molybdenum ratio of 1:12 and a Dawson type heteropolyacid. As the adjusted pH is lower, the proportion of the Keggin type heteropolyacid salt is larger.
When the proportion of each element contained in the precipitate formed from the used catalyst, relative to the amount of each element contained in the used catalyst is defined as the recovery of each element, the recovery of each element varies depending upon the composition of the used catalyst, the amount of ammonium root in the mixture before pH adjustment, and the adjusted pH. In the case of, for example, a used catalyst having a composition of the above-mentioned formula (1), the A element recovered in the form of a heteropolyacid salt of Keggin type is mostly phosphorus. Meanwhile, the A element recovered in the form of a heteropolyacid salt of Dawson type is phosphorus and arsenic; however, when both of them are present, arsenic is recovered preferentially. Therefore, in a used catalyst having a composition containing both phosphorus and arsenic, the adjustment of pH is preferably made to 1.5 or less in order to recover phosphorus preferentially, and to 2 to 6.5 in order to recover arsenic preferentially. In determining the pH to be adjusted, it is desirable to consider the recoveries of individual elements including molybdenum, etc.
In the present invention, the recovery of molybdenum can thus be made preferably 50 mass % or more, more preferably 70 mass % or more. Also, the recovery of the A element can be made preferably 50 mass % or more, more preferably 70 mass % or more.
When the amount of the X element is not sufficient to precipitate a heteropolyacid in the form of a salt of the X element, it is preferred that a raw material for ammonium root is added before the adjustment of pH so that the ammonium root is present in an amount of 0.5 mole or more, preferably 3 to 40 moles per mole of the A element. By this addition, a higher amount of a heteropolyacid can be precipitated in the form of an ammonium salt, and the molybdenum and the A element contained in the precipitate can be recovered at higher recoveries. A higher amount of the ammonium root results in higher recoveries of molybdenum and the A element. The kind of the ammonium root is not particularly restricted as long as it is soluble; and there can be mentioned, for example, ammonia solution, ammonium chloride, ammonium nitrate and ammonium carbonate.
The thus-precipitated compound contains molybdenum, the A element and, further, the X element. The presence of the X element in the compound is desirably a small amount or zero in some cases, depending upon the application of the compound. In such cases, the whole or part of the X element is preferably removed from the mixture before the pH adjustment to 6.5 or less.
There is no particular restriction as to the method for removing the X element. However, there can be mentioned, for example, a method of removing the X ion by its adsorption on an cation exchange resin. As the cation exchange resin, there can be used, for example, a styrene type resin and a chelate resin which are each an ordinary strongly acidic cation exchange resin, and a Na type ion exchange resin is preferred particularly. As to the timing of removal of the X element, there is no particular restriction as long as it is before the pH adjustment to 6.5 or less; however, removal of the X element is preferably made according to the following procedure.
That is, a used catalyst containing at least molybdenum, the A element and the X element is dispersed in water; thereto is added sodium hydroxide for dissolution; as necessary, the resulting insolubles are removed by filtration or the like; the X element is removed using a cation exchange resin or the like; a material for ammonium root is added in an amount of 0.5 mole or more per mole of the A element; then, an acid is added for pH adjustment to 6.5 or less.
There is no particular restriction as to the method for separating the precipitate formed by the pH adjustment, from the liquid containing the precipitate, and there can be mentioned ordinary methods such as filtration (e.g. gravity filtration, pressure filtration, vacuum filtration or filter press), centrifugation and the like. The precipitate may be washed as necessary in order to remove impurities from the precipitate. The solution for this washing is selected in view of the application and solubility of the precipitate, and there can be mentioned, for example, pure water and a dilute aqueous solution of ammonium nitrate, ammonium chloride or the like.
In the present invention, the thus-obtained precipitate is used as a material for catalyst. In this case, the state of the precipitate is not particularly restricted and may be a wet state or a dry state. Also, an oxide obtained by calcining the precipitate may be used when it is desired to use the material for catalyst in-the form of an oxide. The calcination temperature is preferably 200 to 700xc2x0 C.
In the present invention, the process for producing a catalyst is not particularly restricted and can be appropriately selected from various well known processes such as evaporation to dryness, precipitation, oxides mixing and the like, depending upon the state of the precipitate used as a material.
The catalyst produced in the present invention contains at least molybdenum, an A element and an X element. Meanwhile, an ordinary catalyst further on contains other elements so that the composition thereof is suitable for an intended reaction. For example, a catalyst used for production of methacrylic acid by gas phase catalytic oxidation of methacrolein, preferably has a composition of the previously-mentioned formula (1); in producing such a catalyst, materials other than the above-mentioned precipitate are also used. As such materials, there are mentioned oxides, nitrates, carbonates, ammonium salts, halides, etc. of the individual elements constituting the catalyst to be produced. As materials of, for example, molybdenum, there are mentioned ammonium paramolybdate, molybdenum trioxide, etc.
In producing a catalyst, there is first prepared a solution or aqueous slurry containing all materials for catalyst (the solution or slurry is hereinafter referred to as mixed solution). The mixed solution is subjected as necessary to drying, filtration, water content control by heating, or the like. The drying can be conducted by ordinary evaporation to dryness by heating, vacuum drying, air drying, or the like, and the temperature for drying is preferably 60 to 150xc2x0 C.
Next, the thus-obtained mixed solution, water content-controlled mixed solution or dried material is shaped. The shaping in the present invention includes mechanical shaping using an ordinary powder molding machine such as tabletting machine, extrusion molding machine, tumbling granulator or the like; loading shaping of loading a catalyst component(s) on a carrier; drying shaping using a spray dryer, a drum dryer, a slurry dryer or the like; and so forth. There is no particular restriction as to the method for shaping.
The shape after shaping can be determined as desired, and it can be spherical, ring-shaped, columnar, hollow-spherical, flake-like, stellar, etc. As the carrier used in loading shaping, there can be mentioned, for example, inactive carriers such as silica, alumina, silica-alumina, magnesia, titania, silicon carbide and the like. An additive may be added in the shaping. Such an additive includes, for example, organic compounds such as polyvinyl alcohol, carboxymethyl cellulose and the like; inorganic compounds such as graphite, diatomaceous earth and the like; and inorganic fibers such as glass fiber, ceramic fiber, carbon fiber and the like.
The shaped catalyst is then subjected to a heat treatment. The conditions of the heat treatment are not particularly restricted and can be known treatment conditions. For example, in the case of a catalyst for production of methacrylic acid by gas phase catalytic oxidation of methacrolein, the temperature of the heat treatment is preferably 300 to 500xc2x0 C. and it is preferred to conduct the heat treatment in an air stream or in a moisture-controlled air stream.
The catalyst produced according to the process of the present invention may be used by dilution with an inert carrier such as silica, alumina, silica-alumina, magnesia, titania, silicon carbide, stainless steel or the like.
When the catalyst produced according to the present process is used in a reaction, the reaction conditions are not particularly restricted and known reaction conditions can be used. Below are mentioned the reaction conditions when methacrylic acid is produced by gas phase catalytic oxidation of methacrolein.
The concentration of methacrolein in the raw material gas can be varied in a wide range but is preferably 1 to 20% by volume, particularly preferably 3 to 10% by volume. As the oxygen source in the raw material gas, air is economical and oxygen-enriched air may be used as necessary. The oxygen concentration in the raw material gas is preferably 0.3 to 4 moles, particularly preferably 0.4 to 2.5 moles per mole of methacrolein. The material gas may be diluted with an inert gas such as nitrogen, steam, carbon dioxide or the like. The reaction pressure may vary from normal pressure to several atm. The raw material gas may contain a small amount of impurities such as lower saturated aldehydes and the like, and these impurities gives substantially no adverse effect on the reaction. The reaction temperature can be selected in a range of 230 to 450xc2x0 C., particularly preferably in a range of 250 to 400xc2x0 C. The reaction may be conducted in a fixed bed or in a fluidized bed.
By conducting a reaction using a catalyst obtained by the production process of the present invention, it is possible to achieve a conversion which is preferably 90% or more, more preferably 95% or more of the conversion obtained using a virgin catalyst. It is also possible to achieve a selectivity which is preferably 90% or more, more preferably 95% or more of the selectivity obtained using a virgin catalyst. It is further possible to achieve a per-pass yield which is preferably 90% or more, more preferably 95% or more of the per-pass yield obtained using a virgin catalyst.
Hereinafter, the present invention is described using Examples. In the Examples, xe2x80x9cpart(s)xe2x80x9d is/are. part(s) by mass; and the quantitative analyses of contained elements (or molecules) were made by ICP emission spectrometry, atomic absorption spectrometry, ion chromatography and Kjeldahl method. The quantitative analyses of raw material gas and product in production of methacrylic acid were made using gas chromatography. The recovery of each element, the conversion of methacrolein, and the selectivity and per-pass yield of methacrylic acid produced were calculated using the following formulas.
xe2x80x83Recovery (%)=[(mass of element contained in obtained compound)/(mass of element contained in used catalyst)]xc3x97100
Conversion of methacrolein (%)=[(moles of reacted methacrolein)/(moles of fed methacrolein)]xc3x97100
Selectivity of methacrylic acid (%)=[(moles of produced methacrylic acid)/(moles of reacted methacrolein)]xc3x97100
Per-pass yield of methacrylic acid (%)=[(moles of produced methacrylic acid)/(moles of fed methacrolein)]xc3x97100
In 300 parts of pure water were dissolved, at 70xc2x0 C., 63.62 parts of ammonium paramolybdate, 1.05 parts of ammonium metavanadate and 7.61 parts of cesium nitrate. Thereto was added a solution of 3.46 parts of 85 mass % phosphoric acid dissolved in 10 parts of pure water, followed by addition of 1.31 parts of antimony trioxide. The resulting mixture was heated to 95xc2x0 C. with stirring. Then, a solution of 1.45 parts of copper nitrate dissolved in 10 parts of pure water was added, and the resulting mixture was evaporated to dryness with heating and stirring. The solid obtained was dried at 130xc2x0 C. for 16 hours. The dried material was subjected to pressure molding, crushed, and sifted using a sieve to separate a moiety of 0.85 to 1.70 mm. It was heat-treated at 380xc2x0 C. for 5 hours in an air stream to obtain a catalyst. This catalyst had a composition of P1Mo12V0.3Sb0.3Cu0.2Cs1.3.
The catalyst was charged into a reaction tube. Through the reaction tube was passed a mixed gas consisting of 5% by volume of methacrolein, 10% by volume of oxygen, 30% by volume of steam and 55% by volume of nitrogen, at a reaction temperature of 270xc2x0 C. for a contact time of 3.6 seconds to conduct a reaction. As a result, the conversion of methacrolein was 80.8%, the selectivity of methacrylic acid was 81.2% and the per-pass yield of methacrylic acid was 65.6%.